One of X-ray diagnosis apparatuses is a computed tomography (CT) apparatus. The CT apparatus comprises an X-ray tube for radiating X-ray fan beams, and a radiation detector comprising a large number of radiation detection elements. X-ray fan beams radiated from the X-ray tube pass through an object to be measured, and are detected by the radiation detector. The detected data are analyzed by a computer to display a cross section of the object. The measured data are analyzed by a computer to calculate X-ray absorbance at each position in each cross section of the object by a computer, thereby forming an image based on the X-ray absorbance.
As radiation detectors for detecting radiations such as X-rays, etc., radiation detectors comprising radiation-detecting elements obtained by combining ceramic scintillators produced by sintering rare earth oxysulfide powder such as Gd2O2S, Y2O2S, Lu2O2S, etc. comprising Pr, Ce, Eu, Tb, etc. as luminescent elements, and silicon photodiodes have been developed and put into practical use. In the radiation detector comprising ceramic scintillators, radiation-detecting elements can easily be made small to increase the number of channels, thereby obtaining high-resolution image.
In such radiation-detecting elements, when scintillators absorbing radiations emit light with large intensity (luminescence intensity), they have high sensitivity. Diagnosis apparatuses utilizing radiations are recently required strongly to reduce radiations to which humans are exposed. As a result, it has become important to shorten the scanning time. Shorter scanning time from the present level results in shorter integration time in one detecting element, thereby reducing the total amount of radiations absorbed during the integration time. Accordingly, scintillators having high luminescence efficiency (large luminescence intensity) are particularly needed.
JP 2000-313619 A discloses a method for producing rare earth oxysulfide powder used in scintillators, comprising the steps of dispersing at least one rare earth oxide in water, adding 1 mol of sulfuric acid or sulfate corresponding to at least one rare earth to 1 mol of rare earth oxide, calcining the resultant powdery precipitate, and reducing the resultant rare earth oxysulfate.
JP 2004-525848 A discloses a method for producing a high-density, translucent scintillator ceramic, which comprises wet-pulverizing rare earth oxysulfide powder having a specific surface area of at least 10 m2/g in a pulverizing organic liquid to powder having particle sizes of less than 10 μm, forming this powder into a green body having a density of 40-60%, and sintering the green body at a temperature of 1200-1450° C. under atmospheric pressure in vacuum or an inert gas.
Because the rare earth oxysulfide powder obtained by the production method of JP 2000-313619 A contains large particles, high-density sintered bodies cannot be obtained by sintering under normal pressure. To obtain high-density sintered bodies, hot pressing or hot-isostatic pressing should be conducted for sintering, resulting in high cost.
In the production method of JP 2004-525848 A, rare earth oxysulfide is wet-pulverized in a pulverizing organic liquid to adjust its particle sizes, but sulfur is dissociated from rare earth oxysulfide during pulverization, resulting in lattice defects due to the voids of sulfur introduced into rare earth oxysulfide. The lattice defects remain even in a ceramic scintillator obtained by sintering rare earth oxysulfide, so that the ceramic scintillator has poor luminescence intensity.